In the prior art, there is the probability that data wirelessly transmitted, for example, between two electronic instruments are intercepted.
To avoid this problem, one of the two electronic instruments encrypts data which is in turn transmitted to the other electronic instrument. The other electronic instrument then decrypts the encrypted data.
One of known encrypting/decrypting methods is the common-key type cryptographic system in which the encryption key used to encrypt data to be transmitted is identical with the decryption key used to decrypt the encrypted and transmitted data, this single key for encryption and decryption being kept secret. Generally, the Data Encryption Standard (DES) and the Revest-Shamir-Adleman (RSA) system are used as the known encrypting/decrypting methods. The RSA system is a public key cryptosystem in which an encryption key is publicly opened, but a decryption key is kept secret.
When DES is used to perform data transmission between two electronic instruments, both electronic instruments must know previously about the data transmission.
If a plurality of electronic instruments is used to perform the transmission/reception of data, a number of keys are required. More particularly, 45 keys are required by ten electronic instruments.
On the other hand, RSA requires a system for previously registering the public keys. Such a system causes to increase the installation cost or the like. Further, the data processing by RSA is slow.